The present invention relates to a method in connection with the washing of pulp at a chemical pulp mill comprising at least an alkaline cooking process utilizing cooking liquor for producing pulp, brown stock treatment with essentially closed liquid cycles, in which a last washing device is washing device based on pressing of pulp, a press or a washing press, a pulp bleaching plant using ECF-bleaching, wherein chloride-containing effluents are formed, a chemical recovery plant and effluent purification.
The size of pulp mills has grown intensively during the last years, as today a pulp mill producing 1 million ton/a is of normal size and it does not seem that the growth of the size of pulp mills would be ceasing. At the same time that the size of the pulp mills is growing, the mills are being built in areas and surroundings with very strict environmental regulations. For example, the amount of water used by a mill is strongly restricted. Because the size of the mill grows, minor decreases in the water amounts used by the mill per one ton of pulp do not absolutely decrease the amount of water used by the mill, but the amount is compensated back to the same level as the production size increases. This development is difficult especially in countries where the mill simply does not have enough water available or the water resources should be saved for the needs of people and cultivation. In this kind of situation it is simply impossible to build a mill at a place where other demands of production are easily fulfilled, but due to water resources it is not possible to build a mill. Additionally, in many areas a cleaner environment is desired in such a way that the mills produce substances that are less detrimental to the environment. Therefore, it is essential to look for solutions for finding an increasingly closed process.
Chlorine-containing chemicals have been used throughout the production of chemical pulp in several different forms, of which elemental chlorine Cl2, chlorine dioxide ClO2 and hypochlorite NaOCl or CaOCl are the best known. Chlorine-containing chemicals have been used also e.g. in the form of hypochlorous acid in bleaching, but no permanent applications remained in use. On the other hand, the chemical pulp industry desired to tightly maintain a technique in which pulp is bleached with chlorine-containing chemicals so that chlorine dioxide is the main chemical of the bleaching process of the mill. Years-long pressure to reduce the amount of organic chlorine compounds in bleaching effluents has led to the point that first the use of chlorine and hypochlorite was abandoned and further the kappa number of the pulp after digestion was decreased from level 30 to level 10-15 for soft wood and from level 16-20 to level 10-13 for hard wood using an oxygen stage. In 1990s, the aim was to abandon the use of chlorine dioxide as well and many mills switched to the use of total chlorine free (TCF) bleaching technique, wherein the use of chlorine dioxide, too, was replaced by totally chlorine-free bleaching chemicals, such as ozone and peroxide. With this technique, the mills got rid of all chlorine-containing chemicals, but on the other hand many paper producers were unsatisfied with the properties of pulp produced without chlorine chemicals. Therefore, the marginal term for all solutions relating to the closing of the mill is that chlorine dioxide is still used as bleaching chemical.
Thus the dominating position of chlorine dioxide as bleaching chemical has even gained more power during the last years, and not even the latest researches or industrial experiences have managed to destabilize its position, but as a rule the whole pulp industry, with only a few exceptions, has approved the use of chlorine dioxide as the key chemical in bleaching. Thus, if a mill is to further decrease the amount of organic chlorine compounds, the aim of the mills will be, first and foremost, to eliminate them and to treat them inside the mill, rather than to decrease the use of chlorine dioxide.
Modern ECF-bleaching used for bleaching pulp, is typically formed of at least three bleaching stages and three washing apparatuses. In a special case there may be only two washing apparatuses, but such applications are rare. ECF-bleaching covers all such bleaching sequences, which have at least one chlorine dioxide stage and which do not use elemental chlorine in any bleaching stage. Because the use of hypochlorite is due to pulp quality reasons restricted to the production of only a few special pulps, such as dissolving pulps, also hypochlorite is not regarded to be used in the production of ECF-pulp, but it is not totally ruled out. Additionally, the bleaching sequence comprises one alkaline stage, wherein the additional chemicals used are today typically either oxygen, peroxide or both. Further, modern bleachings may use ozone, various types of acid stages and a chelate stage for removing heavy metals. In literature, the bleaching stages are described with letters:
O=oxygen delignification
D=chlorine dioxide stage
H=hypochlorite stage
C=chlorination stage
E=alkaline extraction stage
EO=alkaline extraction stage using oxygen as additional chemical
EO=alkaline extraction stage using peroxide as additional chemical
EOP(PO)=alkaline extraction stage using oxygen and peroxide as additional chemical
P=alkaline peroxide stage
A=acid hydrolysis stage, stage of removal of hexenuronic acids
a=pulp acidation stage
Z=ozone stage
PAA=peracetic acid stage, acid peroxide stage
In this patent application the chemical amount and other amounts are given per one ton of air dry pulp (adt pulp, i.e. air dry metric ton of 90% dry chemical pulp).
When bleaching is called ECF-bleaching, the amount of chlorine dioxide used in the bleaching sequence is more than 5 kg act.Cl/adt pulp. If chlorine dioxide is used in one bleaching stage, most typically the doses are between 5-15 kg act. Cl/adt. The doses refer to active chlorine, whereby when converting to chlorine dioxide the dose has to be divided by a ratio of 2.63.
If the use of peroxide in bleaching is restricted to doses smaller than 6 kg and if chlorine dioxide is the main bleaching chemical, so then the chlorine dioxide dose in the bleaching increases from a level of 25 kg/adt depending on the bleaching properties of the pulp and on how much the kappa number of the pulp has been decreased before starting the bleaching using chlorine-containing chemicals. Thus, the bleaching technique may in view of the process be fairly freely adjusted to various levels of chlorine dioxide consumption so that the amount of chlorine-containing chemicals exiting the bleaching corresponds to the capacity of the chemical cycle to receive chlorides.
In connection with the present invention it is in view of practice most preferable to choose as reference sequence for hard wood a bleaching sequence A/D-EOP-D-P effected with four bleaching stages and leave ozone out. The corresponding sequence for soft wood is D-EOP-D-P. Then the quality of the pulp can be regarded to correspond to the qualities required from ECF-pulp and the pulp yield remains reasonable. Then the chlorine dioxide doses for soft wood are typically between 25-35 kg/adt pulp and for hard wood 20-30 kg/adt. These values can be regarded as measuring values, and there is no need to invent any new specific techniques for bleaching. The theory of bleaching and various connection alternatives render a possibility for countless different bleaching sequences starting from the connection of two washing apparatuses up to six-stage bleaching sequences. At the same time, the number of chlorine dioxide stages may vary from one up to four and therebetween are alkaline stages as appropriate.
When the amount of active chlorine is calculated as described above in form of the chloride amount, it is noted that even with soft wood, for obtaining a good bleaching result, the bleaching line produces about 10 kg of chlorides per one ton of pulp and a hard wood bleaching line even less. If the plant is closed such that less and less of fresh water is led into bleaching, there may be a need to prepare for chlorine dioxide doses of even 50% greater, and on the other hand the amount of chlorides in bleaching effluents increases up to a level of approximately 15 kg, meaning that in practice the greatest doses of active chlorine are 60-70 kg/adt. Values higher than this cannot be considered economically reasonable, but the basic bleaching solution complies with these starting points.
One suggested technique for decreasing the environmental effects of chlorine-containing chemicals is the closing of the liquid cycles of bleaching plants, and modern bleaching plants have reached to a level of 10-15 m3 of effluent/adt pulp without a decrease in pulp quality. Nevertheless, even when decreasing the amount of bleaching effluent from a level of 15 m3/adt pulp to a level of 10 m3/adt an increase in chemical consumption is seen, which thus leads to an ever increasing amount of organic chlorine compounds from bleaching. Thus, a conclusion may be drawn that the closing of the water cycles of bleaching as such does not have a direct influence in the amount of organic chlorine compounds, but on the other hand a smaller amount and a greater concentration of effluents allow for easier and more economical purification thereof.
Chloride-containing chemicals are used in bleaching so that the total chloride dose into the chemical cycle is 5-10 kg of chlorides per one ton of chemical pulp. Because this amount has to be made to pass so that the amount of liquid to be evaporated in the process remains reasonable, the challenge is to find such a process arrangement, where a chloride-containing liquid replaces some other liquid used in a process at the mill. Thus there is no need for separate treatment stages, new non-productive sub-processes at the mill, but the treatment can be carried out by means of existing process stages.
In order to be able to optimize the treatment of a chloride-containing liquid and in practice the treatment of bleaching effluent, it is inevitable to first know the properties of the effluent. In the bleaching, chlorine-containing inorganic compounds and organic chlorine compounds from the reactions of chlorine dioxide or chlorine remain in the process. Bleaching separates from the fibers various compounds of lignin, which remain in the effluent in form of organic molecules. Additionally, sulfuric acid is used in bleaching for pH regulation and as main chemical in the hydrolysis of hexenuronic acids. Sodium hydroxide is also used for pH regulation and lignin extraction in alkaline stages. In addition to these, depending on the bleaching sequence, oxygen and peroxide are used in bleaching, which, however, are in elementary analysis such substances that their contribution in for example purification processes is not noticed. In some special cases, also hydrochloric acid may be used in pH regulation and sulfur dioxide or other reductants in elimination of chemical residuals from the bleaching, i.e. in elimination of unreacted bleaching chemicals.
Closing of the bleaching is based on recycle of filtrates of washing apparatuses from later bleaching stages to preceding stages. The bleaching is planned only for circulating filtrates between bleaching stages and pulp from one stage to another to react with different bleaching chemicals. Thus, closing the whole bleaching is as an idea based on the fact that all substances separated in bleaching end up in filtrates. Optimizing the closing of bleaching is in a great part based on the way how reaction products of bleaching disturb the process of bleaching. Although in many various connections it has been stated that different degrees of closing are possible, practical experience has shown that such washing water arrangements of bleaching where the filtrates are connected so that the amount of effluent is less than 12-13 m3/adt increase the consumption of bleaching chemicals. Naturally, the quality of the pulp and the construction of the bleaching plant dictate the amount of additional chemicals used in the bleaching as the effluent amount of the plant decreases below the above presented level.
Often a research dealing with the closing of bleaching ends in a conclusion that the closing of bleaching succeeds, but the bleaching should be provided with a sink or a kidney in which harmful inorganic substances could be separated from the process. This kind of kidney is often described as a process operating with either membrane technique or ultrafiltration, which again would be a kind of new and separate by-process at the mill. In addition to that, the processes are fairly new and their continuous technical performance has been questioned. As the above-stated is combined with remarkable operational costs, the technology development has not become general.
Thus, partial closing of bleaching and external purification of the generating filtrates (with a volume of 10-15 m3/adt) using e.g. filtration, various known forms of biological treatment, different techniques of chemical treatment and clarification has been regarded as the so-called best available technology for bleaching effluents. After this, the treated water is led back to the water course to the same channel wherefrom the liquid was taken to the mill process or to a different channel. This is in use at both TCF- and ECF-pulp mills. Biological treatment is efficient specifically when the proportion of detrimental organic substances is decreased, which mainly comprise lignin compounds separated in bleaching, hemicelluloses and components originating from extractives, which constitute a significant portion of effluent coming from the bleaching plant. There is an ample amount of various wood-originating compounds, and part of the compounds are chlorinated and part of them are low-molecular compounds of carbon and hydrogen. As microbes act so that they use as nutrition only the organic portion of effluent, all in-organic substances, at least inorganic elements remain in the effluent. Thus, biologically treated effluent has an organic load that makes it clearly cleaner than effluent treated in other ways, but due to the in-organic substances the only choice has been to discharge it from the process.